Lift trucks are commonly used in many industries and under many conditions. They are generally used for elevating and carrying a wide variety of objects from one place to another. Often the lift truck must be parked while the operator or other workers manipulate the load. If the lift truck is parked on level ground, the weight of the lift truck and the frictional force between the tires and ground combine to make the lift truck substantially immovable. However, if the lift truck is parked on an incline, a parking brake must be applied for safe manipulation of the load, or storage of the lift truck while not in use.
Several different types of brakes are provided in the prior art for parking a lift truck. Drum brakes, caliper brakes, hydraulic spring brakes, and various designs of pin brakes are among the types of brakes commonly used. Each of these designs has unsatisfactory characteristics due to its complexity, cost of construction, or high maintenance requirements.
One type of pin brake provided in the prior art has an hydraulic cylinder stroke which pushes the ram or piston shaft through a slotted disc or wheel designed to receive the shaft on a number of positions on the disc or wheel. The slotted disc or wheel may only receive the shaft in a number of discrete positions in its rotation, i.e. when the center of the shaft is in line with the center of the slots or openings of the disc or wheel.
Pin brakes provided in the prior art have at least two significant problems. When the shaft and brake disc are engaged but not precisely aligned, a severe radial load is created on the shaft. This severe radial load causes premature wear of the internal cylinder seals. Also, when the shaft is engaged with the brake disc and the lift truck is parked on an incline, a very high frictional force is created on the shaft by the brake disc. An equally large force is required to retract the shaft and disengage the brake. As a result, very high pressure in the hydraulic unit is required to retract the shaft and disengage the brake. This type of design is complex and expensive to build.
Manually operated pin brakes provided in the prior art have problems similar to hydraulically operated pin brakes. For example, to release a manual parking brake engaged while the lift truck is parked on an incline, the lift operator must simultaneously move the fork lift up the incline by operating the travel mode, and then release the parking brake at the movement the shaft and brake disc are in precise alignment.
Another prior art parking device provides an hydraulic spring brake coupled to a wheel drive motor. The brake is automatically applied by a spring when the travel mode of the truck is arrested and the brake is released hydraulically when the travel mode is resumed. Since this brake requires low operating pressure, a pressure-reducing valve and shuttle valve are necessary to operate the brake properly. Additionally, if the operator arrests the travel mode during operations, the hydraulic spring brake is automatically applied resulting in a jerking motion of the lift. To prevent jerking, a flow control valve must be mounted on the brake return line, thereby adding to the cost and complexity of the brake system.